The present invention relates to a decentered prism optical system and, more particularly, to a decentered prism optical system applicable to an ocular optical system which enlarges a first-order image formed by an objective lens of a microscope, a telescope, etc.
Examples of conventionally known decentered prism optical systems Include those disclosed in Japanese Patent Application Unexamined Publication Number [hereinafter referred to as "JP(A)"] 7-333551 and 8-234137. The present applicant has also proposed decentered prism optical systems in JP(A) 8-320452 and 8-313829. Every one of these known decentered prism optical systems is designed as an image pickup optical system or an ocular optical system, but not corrected for pupil aberration (i.e. aberration concerning the image formation of a pupil transmitted).
In an ocular optical system used in combination with an objective optical system, an exit pupil of an objective lens is projected onto the iris position of an observer's eyeball through the ocular optical system. If the projected pupil is not favorably corrected for pupil aberration, the exit pupil of the ocular optical system is unfavorably aberrated. Consequently, even if the observer's eyeball is placed at the position of the exit pupil of the ocular optical system, a part of the image field of the image for observation is vignetted and thus becomes impossible to observe.